1. Field of the Invention
The present invention relates to a multiplexer/demultiplexer for use in optical-fiber communications and its manufacturing method.
2. Description of the Prior Art
As one of the methods to transmit a plurality of signals in the optical-fiber communications field, there is a transmission technique of wavelength division multiplexing. Since this technique enables such actions as bi-directional communication and the simultaneous transmission of different signals and makes it possible to increase transmission capacity and decrease system costs, it is a powerful technique in the optical-fiber communications field.
In the wavelength division multiplexing transmission, an optical multiplexer for combining a plurality of optical signals having different wavelengths in a single optical fiber on the transmitting side and an optical demultiplexer for separating a received signal into a plurality of optical signals of different wavelengths on the receiving side are very important components.
As the optical multiplexer and optical demultiplexer, devices of the diffraction grating type and of interference film filter type are generally used. In the case where the degree of multiplexing is higher, the diffraction grating type is more useful.
As the optical multiplexers and demultiplexers using diffraction gratings, one type using a planar, linear grating as disclosed in U.S. Pat. No. 4,198,117 and another type using a concave, curved grating disclosed in U.S. Pat. No. 4,387,955 are so far known. Among these, the one using a planar, linear grating has the grating fabricated by a mechanical ruling method or a holographic method by interference of one plane wave and another plane wave, and the other using a concave, curved grating has the grating produced by mechanical ruling or a holographic method by interference of one spherical wave and another spherical wave. However, in these optical multiplexers and optical demultiplexers, the one using the planar, linear grating has a drawback that it requires a part similar to a lens, namely, a light-focusing transmission body, a convergent lens, a collimation lens, and the like as described in U.S. Pat. No. 4,198,117 and its optical system becomes complex. The other one using a concave, curved grating, though not requiring any convergent lens or collimation lens, has a drawback that it is difficult to fabricate since it is shaped in a concave form and therefore it becomes expensive.